Cuba has a higher percentage of doctors than any other country in the world (I think it is something like one doctor to every 230 people), and there are local clinics everywhere. Tell the family you are staying with that you are diabetic and might need to be taken to hospital or have a doctor be called. I am sure they will be able to get you medical assistance if you need it.
Cuba is short of medicines though, even basic things like painkillers, because of the US embargo they have difficulty in buying pharmaceuticals. So it is good that that you are taking the necessary medications.
Cubans do eat a lot of rice, but they also eat a lot of vegetables, pork and fish, so you should be ok for food.
Enjoy the sun and your holiday!